The pressures referred to here are absolute pressures.
The aforementioned adsorption cycles are referred to as transatmospheric. They are carried out in units which generally comprise one or two adsorbers, a compressor or a blower for supplying air, a vacuum pump and, if necessary, an oxygen storage vessel for making the production rate uniform. The blower and the pump are generally of the "Roots" positive-displacement type and may be combined into a single machine in the case of a single-adsorber unit. For the sake of economy and reliability, this or these machine(s) do(es) not generally have a speed variator, and the invention is based on this assumption, and also on that of a substantially constant purity of the oxygen which is produced.
The main parameters of the cycle, in particular the levels of the maximum or high pressure PM and the minimum or low pressure Pm, are determined at the unit's design stage so as to optimize the cost of the oxygen production for a given nominal production rate. Depending on the various economic factors taken into consideration for this optimization, the pressure ratio PM/Pm may range from about 2.0 to values of from 6 to 8.
The invention will be explained below with reference to a two-adsorber unit.
With the two machines processing constant flow rates, if the oxygen demand decreases from the nominal production and the durations of the steps remain unchanged, the effect on the pressures of the adsorbers is small since the oxygen which is produced represents only about 10% of the air flow rate treated. The energy consumed by the machines consequently remains substantially constant, so that the specific energy E.sub.s, which is the energy consumed per m.sup.3 of oxygen produced, increases approximately in inverse proportion to the delivery rate.
For example, if for the nominal production of 100 the specific energy E.sub.s is 100, a production of 80 will give E.sub.s =125 approximately, and a production of 50 will give E.sub.s =200 approximately.
This mode of operation is adopted only if the unit in question operates nearly always at full capacity, or if the cost of the energy is very low.
Furthermore, with this assumption, the purity of the oxygen which is produced improves when the demand decreases.
In order to improve the energy consumption under reduced-load operation, it has been proposed for this type of cycle (EP-A-0 458 350) to add a waiting time or dead time to the basic cycle, during which the machines run idle, that is to say they have their inlets and their outlets in communication with the atmosphere. The cycle is lengthened by the duration of the waiting time, which reduces the number of cycles per hour and therefore the oxygen production of the unit.
Since the machines have low energy consumption during the waiting time, a substantially reduced specific energy E.sub.s is obtained in reduced-load operation in comparison with the previous case.
Thus, with the numerical examples assumed above, for a production of 80, E.sub.s =107 to 112, and for a production of 50, E.sub.s =120 to 130.
This specific energy is, however, still higher than the nominal specific energy because of the extra consumption of the machines during their extended idle operation.